This invention relates to the art of decorating or finishing plastic articles, and more particularly to a new and improved method and apparatus for decorating plastic articles of non-circular cross-section.
One area of use of the present invention is in roll-on hot stamping of plastic articles, although the principles of the present invention can be variously applied. A commonly used process of finishing, i.e. decorating, of plastic parts is roll-on hot stamping, and in this process decorative pigments or metallization, plus protective and adhesive medium, are transferred from a hot stamping foil to the plastic part by hot rolling. Finish of the decoration depends upon temperature, pressure and rolling speed. Constancy of these parameters results in good quality. To achieve this, the plastic part must be moving relative to the roller with constant pressure and constant, linear speed of the point of contact, i.e. area, between the roller and the part.
The foregoing is relatively easily achievable in decorating plastic parts of circular cross-section. On the other hand, decorating parts of non-circular cross-section is relatively more difficult. The majority of existing machines for hot roll-on decoration of the outer periphery of plastic parts having non-circular cross sections are controlled by cam or numerically controlled systems. Machines based on cams have limited range and accuracy. Numerically controlled machines must be programmed for each new plastic part shape, and programming can be complicated and often must be done by the vendor of the machine.
For some configurations of numerically controlled machines, a computer controlled automatic programming method has been developed. In an illustrative prior art method, a part to be printed/stamped is moving in three directions, i.e. linear vertical, linear horizontal and rotational, under a flat horizontally positioned die. During programming, point coordinates of the trajectory are stored in a control unit memory on the basis of signals from a two-state switch indicating contact of the part with a die. This method has a number of disadvantages. In particular, it can be applied only to the flat shape of the stamping die, or to a shape which can be approximated by a flat surface, because the switch can indicate only perpendicular contact force. Also the programming method itself does not guarantee constant pressure necessary for good stamping. Furthermore, during a self programming or learning mode, the stamping machine must be equipped with special devices which replace the standard hot stamping die. Differences in position of the special programming devices and position of the die can result in additional errors. Another disadvantage is that during hot stamping of parts having a cross-sectional configuration comprising sectors of straight lines, it is necessary to apply very high force to have proper pressure when those sectors are stamped. Furthermore, the machine must have three axes of motion implying a complicated and expensive structure. Finally, this prior art approach does not react to variations in dimensions of the parts.